This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The OfType<TResult>(IEnumerable) method returns only those elements in source that can be cast to type TResult. To instead receive an exception if an element cannot be cast to type TResult, use Cast<TResult>(IEnumerable).

This method is one of the few standard query operator methods that can be applied to a collection that has a non-parameterized type, such as an ArrayList. This is because OfType<TResult> extends the type IEnumerable. OfType<TResult> cannot only be applied to collections that are based on the parameterized IEnumerable<T> type, but collections that are based on the non-parameterized IEnumerable type also.

By applying OfType<TResult> to a collection that implements IEnumerable, you gain the ability to query the collection by using the standard query operators. For example, specifying a type argument of Object to OfType<TResult> would return an object of type IEnumerable<Object> in C# or IEnumerable(Of Object) in Visual Basic, to which the standard query operators can be applied.